Archived posting to the Leica Users Group, 2001/02/08

[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]

Subject: [Leica] 1/3-2/3 (Jacques Bilinski)
From: Jim Brick <>
Date: Thu, 08 Feb 2001 07:51:54 -0800
References: <>

The online DOF calculator that I used was obviously flawed. Look at any DOF
chart or even your Leica lens and you will see that DOF extends farther
behind than in front by roughly 1/3-2/3.

To prove this, take a 50mm lens (mine is a Summilux) and set both 2 and 10
meters on f/16. Then read what is opposite the center mark. It will be
slightly over 3 meters. 3+ is not half way between 2 and 10.

Another way is to use a SLR and put a tape measure in front of the lens,
obviously going straight away. Focus on say five feet. Then using DOF
preview, start stopping down and watch what comes into focus. 1/3 front,
2/3 back.

Or buy a good photo book like The Manual of Photography or The Hasselblad
Manual, both of which describe this fact with ample drawings.

I apologize for the apparent error in my example that you quoted below. I
used one of those online DOF calculators and typed in the result without
thinking about it other than the actual depth of the DOF in both cases.



At 09:42 PM 2/7/01 -0800, Jacques Bilinski wrote:
>> A 100mm lens at
>> f/1.4 focused at 20 feet (exact same image size on the film as the 50mm
>> example), the DOF is from 19'7" to 20'5". The depth of field is EXACTLY
>> same. EXACTLY 10 INCHES IN BOTH CASES for the given COC.
>> From the exact point of focus, DOF extends 1/3 forward
>> (toward the camera) and 2/3 back (away from the camera).
>> Jim
>If the second point (1/3, 2/3 etc) is true then why does the d.o.f extend
>exactly 5" forwards and backwards (1/2, 1/2) in the example you gave?  BTW
>the image size of a 50mm lens at 10 feet is NOT the same (by quite a bit)
>as 100mm lens at 20 feet for a 24mm by 36mm negative. And film size DOES
>matter when comparing the coverage angles of a lens.

Replies: Reply from "Henning J. Wulff" <> (Re: [Leica] 1/3-2/3 (Jacques Bilinski; Jim Brick))
In reply to: Message from Jim Brick <> ([Leica] RE: DOF)